MACHINABILITY UNIT II
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denotes the relative relative ease with which which a material
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(usually a metal) metal) can be machined machined using using appropria appropriate te tooling and cutting conditions. •
The term machinability refers to the ease with which a metal can be machined to an acceptable surface finish.
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Materials with good machinability require little power to remove material, achieve cutting at high speed, easily obtain a good finish, and do not cause tool wear.
Definitions of Machi Machinabi nability: lity:
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The first based on material propertie properties s,
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The second based on tool life li fe,, and
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The third based on cutting speed
1. Mac Machin hinabilit ability y is defined defined by the the ease ease or difficu difficulty lty with whic which h the metal can be machined. •
In this light, specific energy, specific horsepower, and shear stress are used as measures, and, in general, the larger the shear stress or specific power values, the more difficult the material is to machine, requiring greater forces and lower speeds.
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In this definition, the material is the key.
2. Machinability is defined de fined by the relative relative cutting speed for a given tool life l ife while cutting some material, compared to a standard material material cut with the same tool material. 3. Cutting speed is measured mea sured by the maximum maxi mum speed at which a tool can provide provide satisfactory performance per formance for for a specified specifie d time time under specified specifi ed conditions. conditions.
4. Other definitions definitions of machinability are based on the ease of rem remov oval al of the chips chips (chip disposal), the quality of the surf surface ace finish of the part part itself, the dimensional stability of the process process,, or the cost to remove a given volume of of me metal. tal.
Criteria used to ev evaluate aluate machina machinability bility 1. Su Surfac rface e finish finish and surfac surface e integ integri rity ty of th the e mac machi hined ned part part.. 2. Tool life fe.. 3. For orce ce and and power power requ requir ired. ed. 4. The le level vel of diffi difficul culty ty in in chip chip con contr trol ol.. •
Good machinability indicates good surface finish and surface integrity, a long tool life, and low force and power requirements.
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As for chip control, and as stated earlier regarding continuous chips, long, thin, stringy, and curled chips can interfere severely with the cutting operation by becoming entangled in the cutting zone.
CRITERIA TO EVALUATE MACHINABILITY
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The relative performance is expressed as an index number, number, called the machinability rating (MR). The base material material used as the standard is given a machinability rating of 1.00. 1.00. B1112 (Free cutting) steel is often used as the base material in machinability comparisons. Materials that are easier to machine than the base have ratings greater than 1.00, and Materials that are more difficult to machine have ratings less than 1.00. Machinability ratings are often expressed as percentages rather than index numbers.
Various conditions of wor work k material material that affect af fect machinability machinability
Physical properties properties of wor work k material material that affect af fect machinability machinability
Cutting parameters affecting af fecting machinability machinability
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A series of tool life tests are conducted on two work materials under identical cutting conditions, varying only speed in the test procedure. The first material, defined as the base material, yields a Taylor Tool Life equation VT0.28 = 350, and the other material (test material) yields a Taylor equation VT0.27 = 440, where speed is in m/min and tool life is in min. Determine the machinability rating of the test material using the cutting speed that provides a 60 min tool life as the basis of comparison. This speed is denoted by V 60
